This invention relates generally to electromagnetic interference (EMI) radiation reduction and in particular to a technique for forming an EMI shield on a glass surface such as used in a Cathode-Ray Tube (CRT), Plasma Display Panel (PDP), Liquid Crystal Display (LCD), Electro-Luminescence (EL), or other display device.
With the increasing proliferation of various electronic appliances such as computer devices and the like, electromagnetic interference becomes a more common phenomenon. EMI is electromagnetic radiation that is emitted and/or received by an electronic device that adversely affects the performance of that device or other devices, or potentially causes harm to persons using the devices. The majority of EMI-related problems occur within a particular band of an electromagnetic spectrum known as the radio frequency (RF) range. Circuits in computers, telecommunication equipment, radios, televisions, medical devices, and numerous other electronic devices both emit RFI and are susceptible to RFI.
Normally, a so-called Faraday cage is provided by a conductive housing that surrounds an emitting or susceptible electronic device. The conductive barrier reflects or absorbs EMI energy and harmlessly carries it to a ground reference potential. In order to accomplish this effect, the housing is made of a conductive material, such as metal, or of a non-conductive material, such as plastic or plastic composite or ceramic, that has been metalized with a conductive coating.
Electronic display devices pose a challenge for EMI design. Testing by numerous private and government organizations, including the National Institute for Occupational Safety and Health (NIOSH) and the Food and Drug Administration""s Bureau or Radiological Health (BRH) have so far failed to show any radiation hazard from the use of video display terminals. For example, the emissions of x-ray and radio frequency radiation have typically been found to be well below public exposure standards set for electronic equipment. Nonetheless, some researchers have suggested that chronic exposure to such radiation may be associated with certain types of health affects such as cancer.
In other applications, it is desirable to reduce EMI radiation from display screens in order to provide a more secure environment. For example, in sensitive industrial and/or military installations, it can be desirable to reduce the electromagnetic radiation from computer equipment in order to prevent eavesdropping activities. In addition, it is known that EMI emissions from display devices can affect the operation of adjacent unshielded electronic devices and may also be detected and read by highly sensitive eavesdropping equipment.
The challenge to provide shielding for a display is caused by the fact that it must include some type of window or other transparent surface. Metallic plates or metal coated plastics cannot therefore be directly used on the front surface of a display, because they would present an opaque material layer that blocks the view of the underlying display.
Various techniques do exist for providing both an electromagnetic shielding effect and visual transparency at the same time. These techniques include woven wire mesh, thin film compositions, and resins.
A woven wire mesh provides an electrically conductive grid or pattern located within or on a transparent base layer. There are several inherent disadvantages of wire mesh and its application to transparent base layers. One problem involves the inconsistency of the quality of the mesh weave. Wire separations, broken wires, and the linearity of the weave are potential problems. There is also, unfortunately, a lack of a variety of materials and configurations that may be used for the wire mesh itself. Mesh is typically available in limited increments such as fifty, eighty, and one hundred openings per inch (opi) and the wire base materials, for example, if chosen to be copper, are typically 0.0022 inches or more in diameter. But these limitations may present unacceptable constraints in certain applications. Such a thin wire mesh is typically flimsy and is difficult and time consuming to apply. The mesh may tend to tear, or otherwise become difficult to retain linearly and/or remain flat against the substrate. Wire meshes also typically present limitations on their overall physical size, especially if mesh, must be electroplated to improve conductivity. The maximum size of such meshes is typically expected to be 24 by 24 inches at the most.
Woven wire mesh being square or rectangular also inherently generate interference patterns on the display. These interference or Moire patterns result from the fact that the mesh and the underlying display pixels are superimposed upon one another. Consistency in the quality of the weave and lack of variety of materials and configurations also greatly contribute to the generation of interference patterns.
Also, electroplating may be applied to the mesh to improve its electro-conductivity or reduce light reflections. Unfortunately, plating can damage the mesh, resulting in inconsistency in the resulting weave. The plating process also adds thickness to the mesh material, resulting in reduced light transmission. However, providing insufficient thickness can result in insufficient electromagnetic shielding performance. Reflection-reducing coatings may be applied to improve visibility. However, these may result in color variation or stains within the completed window.
Another approach is to apply thin film materials to a transparent substrate. Processes such as vapor deposition and other thin film application techniques can be used. Unfortunately, this approach also has inherent disadvantages. First of all is cost. Depending upon the size and coating specified, a vapor deposited window can be ten times, or more, expensive than a wire mesh window. The cost is essentially driven by the high cost associated with vapor deposition equipment, which may require in excess of one million dollars or more to acquire.
Thin film techniques also have size limitations. Indeed, these probably are even more stringent in this regard than the wire mesh approach. Also, it is generally experienced that shielding levels are dramatically lower, typically one-third to one-half as good as the shielding provided by a wire mesh. The problem is that the thin film which exhibit very low resistence (which in turn means high shielding effectiveness) unfortunately typically result in low light transmission.
Yet another approach is to use electro-conductive resin materials. Such resins contain an embedded metallic powder or the like, and can be directly deposited on the transparent base layers. These techniques can be easily used with glass, polycarbonate, and other window materials because the conductive resins are compatible. However, transparency is typically not as good as either the thin film or the wire mesh approach. It is typically expected that shielding is even less effective as well, since the resin is not inherently conductive as a metal would be.
The present invention is a technique for forming an optically transparent electromagnetic shield. The method consists of layering a photo-printable, conductive composition onto an optically transparent substrate. The composition may be applied as a thick film paste in this step.
In a next step, a photo mask is applied over the photo-printable composition and exposed to collimated light. The photo mask may have a fine line width, such as 0.002 inches wide, to define a final grid pattern.
In a next step, a developer is applied to the substrate to remove the non-exposed photo sensitive material.
A final step of firing the substrate makes the remaining composition material conductive, thereby forming the desired conductive grid with the fine line spacing.
Thick film photo-printable compositions are available in a number of different materials and colors. These include various reflective metals such as gold, silver, copper, aluminum, nickel and/or combinations thereof, as well as black metals, or black pigmented materials. In a preferred embodiment, the composition may be a DuPont Fodel(trademark) material.
It may be desirable to construct a multiple layer composition in a combination of initial steps such as forming, for example, a first conductive silver layer followed by a second, or outer, black layer. The second black layer provides for reduction in glare that might otherwise be caused by using only reflective materials.
The photo-printable composition may be applied to the glass substrate as an initial lamination. The steps of the process then proceed as in the case of applying the composition as a thick film paste.
Manufacturing a shielded window by using thick film photo-printable compositions according to the present invention provides a number of advantages.
First, precision high resolution patterning is possible with no resulting distortion. The invention, therefore, provides for a much more consistent quality pattern without risk of broken wires, linearity distortions, or electroplating inconsistencies.
The shield may be provided in an unlimited variety of patterns at essentially no additional cost. The window pattern can therefore be custom tailored for shielding effectiveness, light transmission, and elimination of interference patterns as particular applications may dictate. The only cost associated with changing the conductive pattern is the cost of the photo tool. This provides for great increase in the variety of resulting product configurations, greater control over the elimination of interference patterns on the display, and less cost than a wire mesh approach.
The process is easily adaptable for patterning large areas with high positive yields. Indeed, it can be used to shield displays of sixty inches in diagonal size.
Shielding effectiveness values remain comparable to that achievable with wire mesh. This is because the invention still implements a metal pattern, and does not rely upon conductive resins or other materials that are less effective for conducting electricity. While photo-lithography equipment typically needs to be acquired, the cost of this is in the tens of thousands of dollars as opposed to millions of dollars for thin film implementations.
Furthermore, the invention can be applied efficiently so that expensive materials such as the silver photo-printable composition can be applied efficiently. In particular, an initial course pattern can be silk screened that leaves large areas of the pattern open, with no composition applied. The photo masking step can then be used to provide an ultimately desired thin line widths.
Furthermore, by the additional use of black colored photo-printable compositions in conjunction with metal compositions in a subsequent step, display contrast can be greatly improved by decreasing reflection of external light from the resulting conductive pattern when it is viewed through the front plate of the display.